Flip chip with integrated flux and underfill

ABSTRACT

A flip chip having solder bumps and an integrated flux and underfill, as well as methods for making such a device, is described. The resulting device is well suited for a simple one-step application to a printed circuit board, thereby simplifying flip chip manufacturing processes which heretofore have required separate fluxing and underfilling steps.

FIELD OF THE INVENTION

The present invention relates to a novel flip chip design. Moreparticularly, the present invention relates to a flip chip whichincorporates solder bumps, flux and an underfill material, wherein theflux and underfill are provided by a single material capable ofproviding both flux and underfill properties.

BACKGROUND OF THE INVENTION

In the electronics industry, electrical components such as resisters,capacitors, inductors, transistors, integrated circuits, chip carriersand the like are typically mounted on circuit boards in one of two ways.In the first way, the components are mounted on one side of the boardand leads from the components extend through holes in the board and aresoldered on the opposite side of the board. In the second way, thecomponents are soldered to the same side of the board upon which theyare mounted. These latter devices are said to be “surface-mounted.”

Surface mounting of electronic components is a desirable technique inthat it may be used to fabricate very small circuit structures and inthat it lends itself well to process automation. One family ofsurface-mounted devices, referred to as “flip chips”, comprisesintegrated circuit devices having numerous connecting leads attached topads mounted on the underside of the device. In connection with the useof flip chips, either the circuit board or the chip is provided withsmall bumps or balls of solder (hereafter “bumps” or “solder bumps”)positioned in locations which correspond to the pads on the underside ofeach chip and on the surface of the circuit board. The chip is mountedby (a) placing it in contact with the board such that the solder bumpsbecome sandwiched between the pads on the board and the correspondingpads on the chip; (b) heating the assembly to a point at which thesolder is caused to reflow (i.e., melt); and (c) cooling the assembly.Upon cooling, the solder hardens, thereby mounting the flip chip to theboard's surface. Tolerances in devices using flip chip technology arecritical, as the spacing between individual devices as well as thespacing between the chip and the board is typically very small. Forexample, spacing of such chips from the surface of the board istypically in the range of 0.5-3.0 mil and is expected to approach micronspacing in the near future.

One problem associated with flip chip technology is that the chips, thesolder and the material forming the circuit board often havesignificantly different coefficients of thermal expansion. As a result,differing expansions as the assembly heats during use can cause severestresses, i.e., thermomechanical fatigue, at the chip connections andcan lead to failures which degrade device performance or incapacitatethe device entirely.

In order to minimize thermomechanical fatigue resulting from differentthermal expansions, thermoset epoxies have been used. Specifically,these epoxies are used as an underfill material which surrounds theperiphery of the flip chip and occupies the space beneath the chipbetween the underside of the chip and the board which is not occupied bysolder. Such epoxy systems provide a level of protection by forming aphysical barrier which resists or reduces different expansions among thecomponents of the device.

Improved underfill materials have been developed in which the epoxythermoset material is provided with a silica powder filler. By varyingthe amount of filler material, it is possible to cause the coefficientof thermal expansion of the filled epoxy thermoset to match that of thesolder. In so doing, relative movement between the underside of the flipchip and the solder connections, resulting from their differingcoefficients of thermal expansion, is minimized. Such filled epoxythermosets therefore reduce the likelihood of device failure resultingfrom thermomechanical fatigue during operation of the device.

While underfill has solved the thermal mismatch problem for flip chipson printed circuit boards, it has created significant difficulties inthe manufacturing process. For example, the underfill must be appliedoff-line using special equipment. Typically, the underfill is applied toup to three edges of the assembled flip chip and allowed to flow all theway under the chip. Once the material has flowed to opposite edges andall air has been displaced from under the chip, additional underfill isdispensed to the outer edges so as to form a fillet making all fouredges symmetrical. This improves reliability and appearance. Next, theassembly is baked in an oven to harden the underfill. This process,which may take up to several hours, is necessary to harden and fullycure the underfill. Thus, although the underfill solves the thermalmismatch problem and provides a commercially viable solution, a simplermanufacturing method would be desirable.

Recently, attempts have been made to improve and streamline theunderfill process. One method that has shown some commercial potentialinvolves dispensing underfill before assembling the flip chip to theboard. This method requires that the underfill allow solder jointformation to occur. Soldering of flip chips to printed circuit boards isgenerally accomplished by applying flux to the solder bumps on the flipchip or to the circuit pads on the printed circuit board. Thus, it hasbeen suggested to use an underfill that is dispensed first, prior tomaking solder connections. In order to facilitate solder bonding,however, the underfill must contain flux or have inherent propertiesthat facilitate solder joint formation. Flux is used since the pads onprinted circuit boards often oxidize, and since solder bumps on flipchips are always oxidized. Thus, the flux is designed to remove theoxide layers facilitating solder joint formation.

Certain underfills commonly called “dispense first underfills” have beendesigned with selfcontained flux chemistry. Unfortunately, theproperties required for a good flux and those required for a goodunderfill are not totally compatible. As such, a compromise ofproperties results. The best flux/underfill materials typically requiremore than an hour to harden.

Additionally, flux-containing underfills still require the use ofspecial equipment including robot dispensing machines. Also, sincesolder assembly and underfill application are combined into a singlestep, the flip chip cannot be tested until the assembly is complete.Thus, if the chip does not operate satisfactorily, it cannot be removedbecause the underfill will have hardened, thereby preventing reworking.

Finally, certain problems have been found to arise when applyingflux/underfill materials to bumped surfaces of flip chips. The problemsresult because the rough surface geometry of the bumped surface is notreadily amenable to the application of fluids, particularly those havinghigh viscosity. Thus, providing the flux/underfill directly onto abumped surface raises at least the possibility of discontinuities andair bubbles forming during the flux/underfill application process.Furthermore, by eliminating bumping prior to application of theflux/underfill layer, it may be possible to eliminate process steps,thereby streamlining the manufacturing process while providing chipmakers with greater design and manufacturing flexibility.

In view of the above, a need still exists for a more efficient processthat reduces the need for expensive equipment and that is compatiblewith existing electronic device assembly lines. A need for a reworkableunderfill also exists. A further need exists for a flux/underfillmaterial that can harden quickly while offering both excellent fluxingproperties and excellent underfill properties.

SUMMARY OF THE INVENTION

The present invention relates to an integrated circuit assemblycomprising a semiconductor wafer which includes solder bumps and anunderfill material which also has fluxing properties. In a broad sense,the invention relates to an integrated circuit assembly which includes asubstrate having a plurality of solderable contact sites on one surfaceand a plurality of solder bumps positioned on that surface such thateach of the solderable contact sites has one solder bump associated withand affixed to each solderable contact site. Each site further includesan underfill material which occupies the space defined between each ofthe solder bumps. The underfill material is characterized in that italso offers fluxing properties when heated to the process temperaturesat which chips formed from the wafer are affixed to circuit boards andthe like. Unlike previous methods in which it was desirable to haveportions of the solder bumps extend through the underfill to allow fluxto be applied, in the present invention, the underfill material maycompletely cover the solder bumps since the underfill material itselfacts as a flux.

The present invention also relates to a method for making an integratedcircuit assembly which includes the steps of providing a substratehaving a plurality of solderable contact sites on a surface thereof anda wafer having solder bumps applied to one surface. An underfillmaterial is applied to the surface of the wafer having the solder bumps.The resulting wafer is characterized in that the underfill occupies thespace defined between each of the solder bumps and also covers thebumps, thereby providing a flux on the portion of each solder bump whichwill be contacted with a substrate during chip mounting.

Lastly, the invention relates to a process for affixing a flip chip to acircuit board. The method involves providing a printed circuit boardhaving a plurality of solderable contact sites on a surface, providingan integrated circuit chip of the type described above (i.e., a chiphaving solder bumps and a flux-integrated underfill material present onits surface), and positioning the integrated circuit chip relative tothe printed circuit board such that each solder bump is in contact witha solderable contact site on the printed circuit board. Once positioned,the integrated circuit chip assembly is heated to a temperaturesufficiently high to melt the solder and the underfill material. Moltenportions of the underfill provide fluxing properties for the solder.Subsequently, the assembly is allowed to cool to a temperature whichallows the solder and underfill material to solidify.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a portion of a semiconductorwafer having solder bumps applied to its surface.

FIG. 2 is a schematic representation of a portion of a semiconductorwafer having solder bumps applied to its surface and a flux/underfillmaterial applied over the solder bumps.

DETAILED DESCRIPTION OF THE INVENTION

The present method provides a unique method of applying and underfill toa flip chip wafer. In particular, in the present invention, a coatablesolution of solid materials is applied to a flip chip that has solderbumps already attached to the integrated circuit connection pads. Thematerial is made up of one or more epoxy resins, such as the preferredbisphenol A, and one or more hardeners that have fluxing properties.Such fluxing properties include primarily the ability to reduce metaloxides found on printed circuit conductor pads and on the surface ofsolder and bare metal. The hardeners are selected from the chemicalclasses of carboxylic acids and acid anhydrides. The hardener may be oneor more materials selected from one or both of these classes. Thus, thehardener may be a mixture of several carboxylic acids and several acidanhydrides.

Other additives, such as wetting agents, thixotropic agents, tackifiers,polymerization catalysts, polymerization inhibitors, low levels ofcross-linking agents, conventional fluxes and solvents may also be used.

The underfill material is further provided with a predetermined amountof an appropriate filler to provide the underfill with a coefficient ofthermal expansion (CTE) that approximates that of the solder jointswhich will be formed by the bumps. A mineral filler such as silicondioxide is preferred. The preferred CTE of the resulting underfillmaterial is approximately 25 ppm/° C., although values of up to about 45ppm/° C. are also envisioned. Even after processing the CTE of theunderfill cannot become greater than about 60 ppm/° C., because this cancause detrimental thermomechanical stresses at the solder joints. Thepreferred filler material is spherical and has a diameter less than thehigh of the solder bumps that will be applied to the wafer. Thus, astypical filler ranges in size from about 3 microns to about 15 microns.While silicon dioxide is preferred because of its ready availability,other non-electrically conductive materials such as aluminum nitride,aluminum oxide and beryllium oxide can be use as well.

A solvent, or solvent blend, which is compatible with each of thecomponents is selected. Among the suitable solvents are included manycommon oxygenated, nitrogen-containing solvents as well as many polararomatic solvents. The particular solvent system chosen should haveevaporation and boiling points that allow removal of the solvent in theenvironment of a drying oven once the wafer is coated with the underfillmaterial.

In one embodiment of the present invention, the flux/underfill materialshould provide an underfill which is reworkable following chip mounting.As such, the resulting underfill material must be a thermoplastic or athermoset with a relatively low cross-link density. In contrast,convention underfills are typically highly cross-linked polymers thatcannot be softened, postmounting, to allow removal or reworking of afaulty chip. Of course, while a reworkable underfill is desirable in oneembodiment of the present invention, it should be understood that theinvention is not intended to be limited as such. Rather, a permanentthermoset composition, achieved by adding hardeners that producesubstantial cross-linking may be used as well. Such non-reworkableunderfills offer potential for use in applications in which highoperational temperatures are likely, such as in automotive and aerospaceapplications.

The underfill solution can be formulated to have the correct rheologyfor application to the wafer using any of a number of methods. Forexample, since the ratio of solvent to solids in the solution determinesthe viscosity of the solution, it is possible to formulate underfillsolutions that can be applied using different methods. Since the solventis substantially entirely evaporated after application of the underfillsolution to the wafer, the resulting, solid underfill layer can have thecomposition regardless of the initial viscosity and percent solids ofthe underfill solution. This results because the solvent acts simply asa vehicle for carrying the solids during underfill application.

In one method, the underfill solution can be applied by spin coating, acommon semiconductor processing method in which liquid is deposited ontoa spinning wafer in order to provide a smooth and level coating. Anunderfill having a viscosity in the range of about 80-85 Kcps, measuredat 2.5 RPM using an RVT #6 spindle on a Brookfield viscometer, has beenfound to give good results. When applied to a wafer, a wafer spin rateof about 1200 RPM yields a smooth coating.

A second method is stencil printing. This method requires a more viscousmaterial that is produced using less solvent. The thixotropic index,(i.e., change in viscosity as a result of mechanical shearing), can alsobe adjusted, using thixotropic additives, to improve printingcharacteristics. The thickness of the stencil determines the amount ofmaterial applied to the wafer. That notwithstanding, the stencil shouldbe thicker than the bump height so that the blade applying the underfillmaterial does not contact the bumps. If such contact does occur, damageto the bumps or even displacement of the bumps may occur.

It is preferred that the thickness of the dried underfill material beless than the height of the solder bumps to allow for collapse of thebumps during the attachment process. In one preferred embodiment, thedried underfill material will have a height in the range of about50-80%, and more preferably, about 60-70% that of the bumps. The amountof solvent contained in the underfill solution determines the amount ofthickness reduction that occurs in the underfill during drying andsolvent evacuation. Thus, it is necessary to consider both the stencilthickness and the solvent percent of the underfill solution in order toprecisely control the thickness of the applied underfill. A dryunderfill thickness range of about 25 to about 125 microns is suitableand will depend on the height of the bumps to be produced at a laterstage.

It should be understood that while spin coating and stencil printing arepreferred, many other methods can be used to apply the underfill to thelayer. These include, but are not limited to, needle deposition,spraying, screen printing and others.

Alternatively, the coating composition can be cast onto a release paperand then dried into a film. The resulting, meltable film can be cut intoa proper shape, called a preform, and applied to the wafer. Heating,with the application of pressure, will cause the underfill layer to bondto the wafer. Mild heating would cause the film to melt and bond to thewafer without activating the fluxing properties or causingpolymerization. One advantage of a solid film is that it can be easilyshipped, conveniently stored, and applied by simple mechanical handlingequipment.

Unlike systems which employ a separate flux and underfill, the presentsystem allows the underfill material to cover the solder bumps since itoffers fluxing properties as well as underfill properties. In fact, itis preferred that the material cover the bumps because, in so doing, thebumps will be protected from oxidation, contamination and mechanicaldamage. Each of the application methods described above has thecapability of covering the bumps with the underfill material.

The coating is then dried by heating it in an oven or by direct heatingof the wafer. It has been found to be advantageous to heat the waferwhile simultaneously using a forced hot air oven to help drive solventout of the coating. Combined top and bottom heating can eliminate anytendency to trap solvent in the underfill layer by a process known as“skinning” in which the surface of the underfill material driesprematurely and forms a film (i.e., a skin) that acts as a barrier tofurther solvent evacuation. If drying is carried out properly, theresulting underfill material is non-tacky and amenable to handling.

In some cases, it may be desirable to allow the underfill material tomaintain a slight degree of tackiness. For example, a tacky surface maybe used to hold a chip in position prior to the solder reflow process.In these instances, tackiness may be provided by adding a tackifier tothe composition.

At this stage, the wafer is ready to be diced, or singulated, to produceindividual flip chips. Any of a wide variety of the methods known in theart for dicing wafers can be employed to that end.

The sole requirement of the inventive wafers is that the process be suchthat it does not interfere with the underfill material applied to thewafer/chip surfaces. In one embodiment, dicing can be achieved byattaching the wafer to a holding tape and then sectioning the waferusing a DISCO saw operating at a speed of about 30,000 rpm using a 5micron diamond. Water jet cooling is used to keep the temperature at thecut below the softening point of the film. The individual die or chipcan then be picked off the tape and placed into waffle packs, tape andreel packaging, or other convenient die presentation systems used in theindustry.

Once diced, individual flip chips may now be bonded to circuit boardsand the like. The flip chip is placed and aligned to the bond pads of asubstrate. As used herein, the term “substrate” is intended to mean acircuit board, a chip carrier, another semiconductor device or a metallead frame. It is not necessary to add flux, although flux may be addedfor special reasons such as compensating for excessive oxide onsubstrate pads, or the need to hold the flip chip in place duringassembly.

The positioned chip is then run through a solder reflow line commonlyused for assembly. A multi-zone oven, with individual heat controls thatpermit a heating profile is preferred. The flux melts at a temperatureranging from about 80° C. to about 140° C. The melting point isdetermined by selecting fluxes having epoxy resins with the appropriatemelting point. The flux/hardener, formed of one or more carboxylicacids, one or more acid anhydrides, or a combination of both, reducesoxides present on the solder or the metal surface in contact with thesolder and allows solder joints to form at the substrate pads. Theliquefied flux/underfill also wets the substrate and begins to bond. Asthe ambient temperature increases, (by moving the assembly into hotteroven zones), the flux-hardener reacts with the epoxy resins to form amostly linear, or thermoplastic, polymer with a final softening point ofat least 130° C. and up to about 190° C. The final softening point isdetermined by the melting point of the initial resins and the particulartype or hardener selected. The final temperature should be selected sothat it is not so low that the underfill material softens during deviceuse, nor should it be so high as to result in excessive reworkingtemperatures.

In one embodiment, a small amount of a multifunctional hardener, i.e., across-linking agent, can be added to further increase the softeningpoint of the resulting underfill. It is desirable, however, to keep thesoftening point low enough so that the resulting underfill can still besoftened upon heating to allow the flip chip to be removed. In contrast,if reworkability is not required, and if the work environment of thechip is expected to be subject to high temperatures, a fallthermosetting system can be employed. This can be achieved by adding asubstantial amount of cross-linking agent or through the use ofmultifunctional resins and hardeners.

The heating process, used in the reflow-soldering step, converts thewafer-layer material from a flux to an underfill. The entire processtakes place in the reflow oven. As such, the present invention allowsthe use of standard surface mount technology without the added equipmentor added steps that are required for conventional flip chip underfillprocesses.

Alternatively, a standard flip chip bonder that can apply heat andpressure can be employed instead of the reflow oven. In that embodiment,the flip chip coated with the flux/underfill is placed into contact withthe conductive pads on the circuit board and heat from the bonder headwill activate the flux, form joints by reflowing the solder bumps, andcause the underfill and flux system to bond tightly to the board. Theuse of a standard flip chip bonder would allow a flip chip to beassembled to a board that already contained mounted components. Thismethod could also be used to assemble a chip at a site that is beingreworked.

Reworking is desirable in situations in which a chip mounting step hasfailed to properly position the chip on the board. Specifically, theassembly of fine pitch, high-density components can result inmisalignments and failed connections. Furthermore, since it is difficultto fully test an unpackaged device such as a flip chip, it becomesdesirable to be able to remove the chip if final testing indicates thatthe chip is not operating optimally, either through a fault with thechip or as a result of improper mounting. Thermoset underfills do notallow the assembly to be reworked since thermosets cannot be melted oncethey have crosslinked.

The present invention eliminates the problems associated with thermosetunderfills by incorporating a thermoplastic resin as the main componentof the underfill. Thus, the chip can be removed by raising the chiptemperature to above the melting point of the solder (approximately 183°C. for tin/lead solder) and above the de-bonding temperature of theunderfill resin.

Typically, the rework temperature must be above the solder reflowtemperature, but less than 25 about 220° C. depending on the circuitsubstrate. An average rework temperature would be about 200° C. Thetemperature can be higher if localized heat is used; for example, in analternate embodiment, a chip bonder could be used to remove chips from asubstrate post-bonding. In still another embodiment, the underfill mayalso include a B-staged thermoset that will de-polymerize at an elevatedtemperature.

The invention can be further understood with reference to the attachedFigures. As can be seen schematically in FIG. 1, a semiconductor device10 comprises a portion of a semiconductor wafer 12 having solder bumps14 applied to its surface. Subsequently, as represented schematically inFIG. 2, the device 10 has had a flux/underfill material 16 applied tothe surface of the wafer 12 having the solder bumps 14. The underfillmaterial 16 occupies at least the spaces between the bumps 14 and alsocovers the bumps.

The following Examples will help to illustrate the invention further.

EXAMPLES Exarnle 1

Flux/underfill Preparation

20.4% by weight bisphenol A epoxy resin (Ciba, GT7074) was blended with24.4% by weight dipropylene glycol methyl ether acetate (Dow). 0.7% byweight polyamide thixotropic agent (King Industries, Disparlon 6650) wasdispersed in the epoxy resin solution at 65° C. for 15 minutes. Theblend was cooled to 25° C. and 5.4% by weight 1,4-cyclohexanedicarboxylic acid, 0.4% by weight 2,4,6-triamino pyrimidine, 48.5% byweight 5-micron silica filler (LE-05, from Tatsumori Ltd., Tokyo,Japan), and 0.2% by weight epoxy silane (TS-100 from OSI Specialties,Friendly, W.V.), were dispersed in the blend at high shear.

Example 2

Flux/Underfill Preparation

This material was prepared in a manner similar to that of Example 1,however, 25% by weight bisphenol A and 20% by weight dipropylene glycolmethyl ether acetate, were substituted for those amounts provided above.2% by weight hydrogenated castor oil was substituted for the polyamidethixotropic agent. Additionally, following the cooling step, theadditives of Example 1 were substituted with the following: 10% byweight adipic acid, 2% by weight 2,4,6-triamino pyrimidine, 40.9% byweight 5-micron silica filler and 0.1% by weight silane.

Example 3

Flux/Underfill Preparation

This material was prepared in a manner similar to that of Example 1,however, 40% by weight bisphenol A epoxy resin (Shell, Epon 1007F) and45% by weight dipropylene glycol methyl ether acetate, were substitutedfor those amounts provided above. 5% by weight hydrogenated castor oilwas substituted for the polyamide thixotropic agent. Additionally,following the cooling step, 10% by weight adipic acid was added.

EQUIVALENTS

From the foregoing detailed description of the specific embodiments ofthe invention, it should be apparent that a unique flip chip having anintegrated flux and underfill has been described. Although particularembodiments have been disclosed herein in detail, this has been done byway of example for purposes of illustration only, and is not intended tobe limiting with respect to the scope of the appended claims whichfollow. In particular, it is contemplated by the inventor that varioussubstitutions, alterations, and modifications may be made to theinvention without departing from the spirit and scope of the inventionas defined by the claims.

What is claimed is:
 1. An integrated circuit assembly which comprises:a) an integrated circuit substrate having a top surface and anelectrical contact surface, the electrical contact surface having aplurality of solderable contact sites on a surface thereof; b) aplurality of solder bumps positioned on the electrical contact surfaceof the substrate such that each of the solderable contact sites has onesolder bump associated therewith, the solder bumps being affixed to thesolderable contact sites; and c) a thermoplastic underfill materialapplied to the electrical contact surface of the integrated circuitsubstrate prior to positioning the assembly on a circuit board, thethermoplastic underfill material at least occupying a space definedbetween each of the solder bumps and characterized in that, upon heatingto a solder reflow temperature, at least a portion of the underfillmaterial acts as a solder flux.
 2. The integrated circuit assembly ofclaim 1 wherein the substrate comprises a semiconductor wafer.
 3. Theintegrated circuit assembly of claim 2 wherein the substrate comprises asemiconductor chip.
 4. The integrated circuit assembly of claim 3wherein the substrate comprises a flip chip.
 5. The integrated circuitassembly of claim 1 wherein the underfill material covers substantiallyall of each solder bump.
 6. The integrated circuit assembly of claim 1wherein the underfill material comprises an epoxy resin and a materialselected from the group consisting of carboxylic acids, anhydrides andcombinations thereof.
 7. The integrated circuit assembly of claim 6wherein the underfill material comprises an epoxy resin and a materialselected from the group consisting of adipic acids and cyclohexanedicarboxylic acids.
 8. The integrated circuit assembly of claim 1wherein the underfill material is reworkable.
 9. The integrated circuitassembly of claim 1 wherein the thermoplastic material is selected fromthe group consisting of phenoxy resins, acrylic resins, methacrylicresins, polycarbonate resins, polyamide resins, polybutene resins,polyester resins, polyolefin resins and mixtures thereof.